This invention relates to network analysis and, more particularly, to error correction of network analyzers employed for electrical circuit measurements. Specifically, one embodiment of the invention provides a very short length of precision coaxial transmission line, connected by a fixture to at least one test port of a high performance microwave vector network analyzer, as a standard for error correction, or calibration, and the use of this standard for error-correcting, or calibrating, such a network analyzer.
Network analyzers are universally used for determining response characteristics of various devices under test, such as filter circuits (or more complex electrical circuits), in order to characterize the device or test it to assure that it meets specifications. The devices under test are either one-port or two-port circuits. The accuracy of a network analyzer depends not only upon the design of the analyzer, but also upon calibration of the analyzer.
Accordingly, various one-port and two-port techniques have been developed for calibrating network analyzers. Known calibration techniques vary in complexity and accuracy.
Typically, these calibration techniques have involved the use of open-, short-, and load-circuit electrical measurements. In traditional one-port calibration techniques, the test port of the network analyzer is open-circuited, and a measurement is taken. This process is repeated with the test port short-circuited, and finally with an impedance-matched load connected across the test port. These measurements are then utilized to calibrate the network analyzer. In traditional two-port calibration techniques, these same measurements are taken for each test port of the network analyzer individually, and, additionally, another measurement is taken by connecting the two test ports together to measure transmission. Often, these techniques employ sliding loads and/or transmission lines that have lengths that are long relative to the wavelengths at the measurement frequencies of interest.
When a length of transmission line is used in connection with the calibration technique, the electrical measurement depends upon the mechanical accuracy of the transmission line. The challenge is therefore to construct a precision transmission line having a known fixed characteristic impedance (Z.sub.0) when inserted into a network analyzer measurement configuration between the test port or ports of the analyzer, on the one hand, and, in some instances, a load or short, on the other hand. This characteristic impedance should be accurately known to many decimal places.
Until recently, the largest contributor to network analyzer measurement uncertainty has been the lack of accurate and traceable calibration standards. The second largest contributor has been the poor repeatability of the connectors on the standards and the test ports calibrated by them.
In this latter regard, problems have arisen with the connection of the inner conductor of known precision coaxial transmission lines. Heretofore, contact between the inner conductors of the coaxial transmission line and the network analyzer test port, or load or short, has been effected by collets or a pin-in-socket connection.
One known coaxial transmission line, available from various manufacturers, comprises an inner conductor rod having recesses formed in the ends. A collet, such as an APC-7 (trademark of Amphenol Corporation) snowflake, is inserted into the recess at each end of the conductor rod. The inner conductor is supported by insulators, or beads, typically disposed near the ends of the conductor rod in the interstitial space between the rod and the barrel of the surrounding coaxial outer conductor. Unfortunately, the insulators adversely affect performance. Also, connections are not highly repeatable due to the variable compliance of the collets when the conductor rod is interfaced with similar collets associated with the mating inner conductors of the network analyzer test port and load or short.
Another known coaxial transmission line, for example, included in the Model Number HP 11637A calibration kit, available from Hewlett-Packard Company of Palo Alto, California, comprises an inner conductor rod which is not supported by insulators. The rod is shorter than the outer conductor, and also shorter than the distance to be spanned by the inner conductor. A pin is formed at each end of the rod and extends outwardly away from the rod end into contact with a socket provided in the mating inner conductor of the network analyzer test port or load or short, as the case may be, opposite the rod end. The outer conductor surrounds the inner conductor and is typically provided with a threaded collar for engagement with a threaded sleeve extending from the test port or load or short. Unfortunately, the rod can be radially offset from the centerline of the structure to which the coaxial transmission line is connected, the conductor rod can be longitudinally shifted with respect to the outer conductor, or the inner conductor can bow.
An improved pin-in-socket connection is found in the Model Number 2653, available from Maury Microwave Corporation of Cucamonga, California, in which contact between the inner conductors is effected by a spring mechanism. The ends of the rod are provided with recesses. A spring-loaded pin resides in the recess at each end of the rod and is biased outwardly away from the rod end into contact with a socket provided in the mating inner conductor of the network analyzer test port or load or short opposite the rod end. Additionally, the inner conductor rod has conical ends, and countersinks are provided in the mating inner conductors of the network analyzer test port, load, or short to aid in aligning the inner conductors, but axial shift and bowing problems persist.
These misalignment problems and asymmetries result in irreproducibility of connections required for accurate network analyzer calibration. Therefore, it is desirable to provide a more accurate and repeatable connection between the coaxial transmission line and the network analyzer test port, load, or short.
However, the optimum length of the coaxial transmission line is typically calculated based upon the required frequency of operation for the calibration. In some cases, the desired length is very short.
When conventional interface dimensions of pin-in-socket connectors are examined, the desired length for the coaxial transmission line can be exceeded when a conventional male and female coupling mechanism is used. The typical coupling dimensions of the pin-in-socket connector make the outer conductor physically impossible to construct.
Considered in more detail, a cross-sectional view of the construction of a known coaxial airline with complete male and female connectors on the outer conductor is shown in FIG. 1. The conventional coaxial airline shown in FIG. 1 has to be long enough to accommodate a male nut, with it's snap ring, and the threads of the coaxial airline at the female end connected to the test port. With no length available for the standard coupling features if the coaxial airline were to have a shorter length, connection to effect calibration would not be possible. It is therefore desirable to overcome the limitations imposed on the length of the coaxial airline used for calibration.
A precision coaxial transmission line in the form of a very short coaxial airline is disclosed in copending U.S. patent application SC/Ser. No. 07/099,434, filed on Sept. 21, 1987, now U.S. Pat. No. 4,845,423, issued on July 4, 1989, and assigned to the same assignee as this patent application. The short airline is approximately one-quarter wavelength at the middle frequency of the measurement frequency range over which the network analyzer is calibrated. The inner and outer conductors are separate. The inner conductor is threadedly connected to a mating inner conductor, such as the inner conductor of the network analyzer test port. For reliable interface at the other end of the inner conductor, a precision coaxial connector is preferably provided for connection to the inner conductor of a device under test. The outer conductor of the short coaxial airline is then attached coaxially with the inner conductor between the network analyzer test port and the device under test. The lengths of the inner and outer conductors of the short coaxial airline are identical. Because the inner and outer conductors are the identical length and the coaxial airline is short, for example, 7 mm, the inner conductor is not bowed. Also, the inner conductor is properly centered. Nevertheless, the use of this threaded connection with the inner conductor can require modification of the center conductor of the test port. Also, the repeatability of the connection at the other end of the inner conductor is not substantially improved.